Compositions For Enhanced Fracture Cleanup Using Redox Treatment

ABSTRACT

A cleanup fluid for reducing a viscosity of a residual viscous material in fractures of a hydrocarbon-bearing formation is disclosed. The cleanup fluid includes an acid precursor, the acid precursor operable to trigger an exothermic reaction component and the exothermic reaction component operable to generate heat, where the heat is operable to reduce a viscosity of the residual viscous material to create a reduced viscosity material, the reduced viscosity material operable to flow from the fractures.

PRIORITY

This application is a continuation-in-part application of and claimspriority to and the benefit of U.S. non-provisional patent applicationSer. No. 14/689,901, filed Apr. 17, 2015, which claims priority to andthe benefit of U.S. Provisional Patent Application No. 61/980,664, filedApr. 17, 2014, the entire disclosures of which are hereby expresslyincorporated here by reference.

FIELD

This disclosure relates to a composition and method to improve therecovery of hydrocarbons from a fractured formation. More specifically,this disclosure relates to a composition and method to reduce theviscosity of a fracturing fluid or other undesired viscous fluid presentin a wellbore or hydrocarbon-bearing reservoir.

BACKGROUND

Hydraulic fracturing fluids containing proppants are used extensively toenhance productivity from hydrocarbon-bearing reservoir formations,including carbonate and sandstone formations. During hydraulicfracturing operations, a fracturing treatment fluid is pumped under apressure and rate sufficient for cracking the formation of the reservoirand creating a fracture. Fracturing operations usually consist of threemain stages including a pad fluid stage, a proppant fluid stage, and anoverflush fluid stage. The pad fluid stage typically consists of pumpinga pad fluid into the formation. The pad fluid is a viscous gelled fluidwhich initiates and propagates the fractures. Auxiliary fractures canpropagate from the fractures to create fracture networks. A fracturenetwork can comprise fractures and auxiliary fractures. Auxiliaryfractures can connect the fractures.

The proppant fluid stage involves pumping a proppant fluid into thefractures of the formation. The proppant fluid contains proppants mixedwith a viscous gelled fluid or a visco-elastic surfactant fluid. Theproppants in the proppant fluid are lodged in the fractures and createconductive fractures through which hydrocarbons flow. The final stage,the overflush stage, includes pumping a viscous, gelled fluid into thefractures to ensure the proppant fluid is pushed inside the fractures.While the three stages have different aims, all three make use of highlyviscous fluids, in addition to or alternative to gelled fluids, toachieve those aims.

A downside of the traditional method is that a high volume of gelled orpolymeric materials can be left behind in the fractures. The gelledmaterials can be concentrated around the proppant in the fractures orcan be freely located in the fractures. The gelled material acts toblock the fractures reducing the fracture conductivity. The hydrocarbonswhich flow from the reservoir formation are unable to move the gelledmaterials. Traditional methods for cleaning the fractures involveviscosity breakers or other elements to breakdown the viscous fracturingfluids. These traditional methods suffer from an inability to completelycleanup the fractures, leaving residual viscous material and reducedconductivity.

SUMMARY

This disclosure relates to a composition and method to improve therecovery of hydrocarbons from a fractured formation. More specifically,this disclosure relates to a composition and method to reduce theviscosity of a fracturing fluid or a blockage material, such as, forexample, a gelled or viscous fracturing fluid, or asphaltenes, or asimilar oily sludge.

In one aspect, a method for improved hydrocarbon recovery from aformation due to cleanup of a residual viscous material is provided. Themethod includes the step of fracturing the formation with a fracturingfluid to generate fractures. The fracturing fluid includes a viscousfluid component, the viscous fluid component operable to fracture theformation to create fractures leaving behind the residual viscousmaterial in the fractures, the viscous fluid component having aviscosity, a proppant component, the proppant component includes aproppant, the proppant operable to hold open the fractures, where theproppant component is carried to the fractures by the viscous fluidcomponent, and a cleanup fluid.

The cleanup fluid includes an acid precursor, the acid precursoroperable to trigger an exothermic reaction component, and the exothermicreaction component operable to generate heat, where the heat is operableto reduce a viscosity of the residual viscous material to create areduced viscosity material, the reduced viscosity material operable toflow from the formation. Fractures can include auxiliary fractures,which propagate from the fractures.

In certain aspects, the exothermic reaction component includes anammonium containing compound and a nitrite containing compound. Incertain aspects of the present disclosure, the ammonium containingcompound is NH₄Cl and the nitrite containing compound is NaNO₂. Incertain aspects of the disclosure, the acid precursor is triacetin.

In a second aspect of the present disclosure, a cleanup fluid forreducing a viscosity of a residual viscous material in fractures isprovided. The cleanup fluid includes an acid precursor, the acidprecursor operable to trigger an exothermic reaction component, and theexothermic reaction component operable to generate heat, where the heatis operable to reduce a viscosity of the residual viscous material tocreate a reduced viscosity material, the reduced viscosity materialoperable to flow from the fractures.

In certain aspects, the exothermic reaction component includes anammonium containing compound and a nitrite containing compound. Incertain aspects of the present disclosure, the ammonium containingcompound is NH₄Cl and the nitrite containing compound is NaNO₂. Incertain aspects of the present disclosure, the acid precursor istriacetin.

In a third aspect, a method to cleanup fractures post hydraulicfracturing is provided. The method includes the steps of fracturing aformation in a hydraulic fracturing operation to produce fractures, andinjecting a cleanup fluid into the fractures to reduce a viscosity of aresidual viscous material.

In certain aspects of the present disclosure, the step of fracturing theformation includes the step of fracturing the formation with afracturing fluid to generate fractures. The fracturing fluid includes aviscous fluid component, the viscous fluid component operable tofracture the formation to create fractures leaving behind the residualviscous material in the fractures, the viscous fluid component having aviscosity, and a proppant component, the proppant component comprising aproppant, the proppant operable to hold open the fractures, where theproppant component is carried to the fractures by the viscous fluidcomponent. In certain aspects of the present disclosure, the cleanupfluid includes an acid precursor, the acid precursor operable to triggeran exothermic reaction component, and the exothermic reaction componentoperable to generate heat, where the heat is operable to reduce aviscosity of the residual viscous material to create a reduced viscositymaterial, the reduced viscosity material operable to flow from thefractures. In certain aspects of the present disclosure, the exothermicreaction component includes an ammonium containing compound and anitrite containing compound. In certain aspects, the ammonium containingcompound is NH₄Cl and the nitrite containing compound is NaNO₂. Incertain aspects, the acid precursor is triacetin.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with regard to the followingdescriptions, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of thedisclosure and are therefore not to be considered limiting of thedisclosure's scope as it can admit to other equally effectiveembodiments.

FIG. 1 is a graphic representation of the effect of the cleanup fluid onthe viscosity of a residual viscous material.

FIG. 2 is a graphic representation of the heat and pressure generated byan exothermic reaction component.

FIG. 3a is a pictorial representation of the residual viscous materialbefore the reaction of an exothermic reaction component of the cleanupfluid.

FIG. 3b is a pictorial representation of the residual viscous materialafter the reaction of an exothermic reaction component of the cleanupfluid.

FIG. 4 is a graphic representation of the effect of the reaction of theexothermic reaction component on the viscosity of a fracturing fluid.

FIG. 5a is a pictorial representation of blockage materials collectedfrom an injection well.

FIG. 5b is a pictorial representation of blockage materials collectedfrom an injection well.

FIG. 6 is a graph showing the effect of an exothermic reaction componenton the temperature of blockage materials from the injection well inExample 4.

FIG. 7 is a graph showing the effect of an exothermic reaction componenton the viscosity of blockage materials from the injection well inExample 4.

FIG. 8 is a graph showing the effect of an exothermic reaction treatmentused to treat the blocked wellbore of Example 4.

FIG. 9 is a graph showing the effect of an exothermic reaction treatmentused to treat the rock matrix surrounding the wellbore of Example 4.

FIG. 10 is a graph showing the pre-exothermic reaction componentinjectivity and post-exothermic reaction component injectivity of theinjection well of Example 4.

DETAILED DESCRIPTION

While the disclosure will be described with several embodiments, it isunderstood that one of ordinary skill in the relevant art willappreciate that many examples, variations and alterations to theapparatus and methods described here are within the scope and spirit ofthe disclosure. Accordingly, the embodiments described here are setforth without any loss of generality, and without imposing limitations,on the claims.

In one aspect, a method for improved hydrocarbon recovery from aformation due to cleanup of a residual viscous material is provided. Thehydraulic fracturing operation fractures the formation using fracturingfluid to create fractures. Formations include sandstone and carbonate,for example.

The fracturing fluid includes a viscous fluid component and a proppantcomponent. The viscous fluid component has a viscosity. The viscousfluid component is operable to increase the viscosity of the fracturingfluid. Viscous fluid components include viscosified water-based fluids,non-viscosified water-based fluids, gel-based fluids, gel oil-basedfluids, acid-based fluids, and foam fluids. Gel-based fluids includecellulose derivatives and guar-based fluids. Cellulose derivativesinclude carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxypropyl cellulose, and methyl hydroxylethyl cellulose.

Guar-based fluids include hydroxypropyl guar, carboxymethyl guar, guarcross-linked boron ions from an aqueous borax/boric acid solution, andguar cross-linked with organometallic compounds. Organometalliccompounds include zirconium, chromium, antimony, and titanium salts. Geloil-based fluids include aluminum phosphate-ester oil gels. In at leastone embodiment of the present disclosure, the viscous fluid component isan aqueous guar solution, having a concentration of guar gum betweenabout 0.1% and about 15%, between about 0.1% and about 10%, betweenabout 1% and about 10%, between about 2% and about 8%, and between about4% and about 6%.

The proppant component includes a proppant. The proppant is operable tohold open fractures created by the viscous fluid component. Anyproppants capable of holding open fractures to create a conductivefractures are suitable for use in the present disclosure. In someembodiments, the proppant component includes a viscous carrier fluidhaving a viscosity. Viscous carrier fluids include viscosifiedwater-based fluids, non-viscosified water-based fluids, gel-basedfluids, gel oil-based fluids, acid-based fluids, and foam fluids.Gel-based fluids include cellulose derivatives and guar-based fluids.Cellulose derivatives include carboxymethyl cellulose, hydroxyethylcellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropylcellulose, and methyl hydroxyl ethyl cellulose.

Guar-based fluids include hydroxypropyl guar, carboxymethyl guar, guarcross-linked boron ions from an aqueous borax/boric acid solution, andguar cross-linked with organometallic compounds. Organometalliccompounds include zirconium, chromium, antimony, and titanium salts. Geloil-based fluids include aluminum phosphate-ester oil gels. In someembodiments, the hydraulic fracturing operation uses a one stagefracturing fluid, in which the fracturing fluid includes both theviscous fluid component and the proppant component, in which the viscousfluid component carries the proppant component to the fractures.

In at least one embodiment, the hydraulic fracturing operation uses amulti-stage fracturing fluid in which the viscous fluid component isinjected into the formation, followed by the proppant component in theviscous carrier fluid. In some embodiments, the injection of theproppant component is followed by injection of additional viscous fluidsto ensure the proppants are placed in the fractures. The additionalviscous fluids have a viscosity.

In some embodiments, the viscosity of the viscous fluid component, theviscous carrier fluid, and additional viscous fluids are the same. Insome embodiments, the viscosity of the viscous fluid component, theviscous carrier fluid, and additional viscous fluids are different. Theinjection of the fracturing fluid ceases after the proppants are placedin the fractures and the fracturing fluid is allowed to seep from thefractures. In some embodiments, the injection of the hydraulicfracturing fluid including the viscous fluid component in addition to oralternative to the proppant component in addition to or alternative tothe overflush component in addition to or alternative to the exothermicreaction component does not generate foam or introduce foam into thehydraulic formation including the hydraulic fractures.

The hydraulic fracturing operation can leave residual viscous materialin the fractures of a hydraulic formation. Residual viscous materialscan include carboxymethyl cellulose, hydroxyethyl cellulose,carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methylhydroxyl ethyl cellulose, guar gum, hydroxypropyl guar, carboxymethylguar, guar cross-linked with boron, aluminum phosphate-ester oil gel,and guar cross-linked with organometallic compounds. Organometalliccompounds include zirconium, chromium, antimony, and titanium salts. Insome embodiments of the present disclosure, the residual viscousmaterial is a gelled material. In some embodiments of the presentdisclosure, the residual viscous material is a polymeric material. In atleast one embodiment of the present disclosure, the residual viscousmaterial is guar gum. The residual viscous material has a viscositygreater than the fracturing fluid. In at least one embodiment of thepresent disclosure, the residual viscous material is surrounding oradjacent to the proppants placed in the fractures.

The cleanup fluid acts, after the proppants have been placed in thefractures, to remove the residual viscous material. In one embodiment,the cleanup fluid is mixed with the fracturing fluid. In at least oneembodiment of the present disclosure, where a multi-stage fracturingfluid is used, the cleanup fluid is a component of the fluids used ateach stage of the hydraulic fracturing operation. In an alternateembodiment, the cleanup fluid is added only to the fluid of the finalstage of the hydraulic fracturing operation, such as, for example, theoverflush stage. In some embodiments, the cleanup fluid is pumped to thefractured formation as a separate step following the hydraulicfracturing operation.

In some embodiments, the cleanup fluid includes an acid precursor and anexothermic reaction component. The reaction of the exothermic reactioncomponent results in a release of kinetic energy and thermal energy. Thereaction of the exothermic reaction component generates heat andincreases the pressure. The generated heat increases the temperature ofthe surrounding fluids, including fracturing fluid remaining in thefractures and residual viscous material. The increase in temperaturereduces the viscosity of the fracturing fluid. The increase intemperature reduces the viscosity of the residual viscous material leftin the fractures to create a reduced viscosity material. The reducedviscosity material flows from the fractures of the formation to thewellbore. The increase in pressure provides lift energy to push thereduced viscosity materials through the wellbore toward the surface. Theremoval of the residual viscous material increases the conductivity ofthe fractures. Increased conductivity of the fractures increases seepageof the fracturing fluid, improves fracturing efficiency, minimizes needfor additional fracturing jobs, minimizes time between fracturing andwell production, and increases hydrocarbon flow, which translates toincreased hydrocarbon recovery.

The acid precursor is any acid that releases hydrogen ions to triggerthe reaction of the exothermic reaction component. Acid precursorsinclude triacetin (1,2,3-triacetoxypropane), methyl acetate, HCl, andacetic acid. In at least one embodiment, the acid precursor istriacetin. In at least one embodiment, the acid precursor is aceticacid.

The exothermic reaction component includes one or more redox reactantsthat exothermically react to produce heat and increase pressure.Exothermic reaction components include urea, sodium hypochlorite,ammonium containing compounds, and nitrite containing compounds. In atleast one embodiment of the present disclosure, the exothermic reactioncomponent includes ammonium containing compounds. Ammonium containingcompounds include ammonium chloride, ammonium bromide, ammonium nitrate,ammonium sulfate, ammonium carbonate, and ammonium hydroxide.

In at least one embodiment, the exothermic reaction component includesnitrite containing compounds. Nitrite containing compounds includesodium nitrite and potassium nitrite. In at least one embodiment, theexothermic reaction component includes both ammonium containingcompounds and nitrite containing compounds. In at least one embodiment,the ammonium containing compound is ammonium chloride, NH₄Cl. In atleast one embodiment, the nitrite containing compound is sodium nitrite,NaNO₂.

In at least one embodiment of the present disclosure, the exothermicreaction component includes two redox reactants: NH₄Cl and NaNO₂, whichreact according to the following:

In a reaction of the exothermic reaction components according to theprevious equation, generated gas and heat contribute to the reduction ofthe viscosity of the residual viscous material.

The exothermic reaction component is triggered to react. In at least oneembodiment of the present disclosure, the exothermic reaction componentis triggered within the fractures. In at least one embodiment of thepresent disclosure, the acid precursor triggers the exothermic reactioncomponent to react by releasing hydrogen ions.

In at least one embodiment, the exothermic reaction component istriggered by heat. The wellbore temperature is reduced during a pre-padinjection or a pre-flush with brine and reaches a temperature less than120° F. (48.9° C.). The fracturing fluid of the present disclosure isthen injected into the well and the wellbore temperature increases. Whenthe wellbore temperatures reaches a temperature greater than or equal to120° F., the reaction of the redox reactants is triggered. In at leastone embodiment, the reaction of the redox reactants is triggered bytemperature in the absence of the acid precursor. In at least oneembodiment, the exothermic reaction component is triggered by heat whenthe exothermic reaction component is within the fractures.

In at least one embodiment, the exothermic reaction component istriggered by pH. A base is added to the fracturing fluid of the presentdisclosure to adjust the pH to between 9 and 12. In at least oneembodiment, the base is potassium hydroxide. The fracturing fluid withthe base is injected into the formation. Following the injection of thefracturing fluid, an acid is injected to adjust the pH to less than 6.When the pH is less than 6, the reaction of the redox reactants istriggered. In at least one embodiment, the exothermic reaction componentis triggered by pH when the exothermic reaction component is within thefractures.

In at least one embodiment of the present disclosure, the cleanup fluidis introduced to the fractures following the hydraulic fracturingoperation. Dual-string coiled tubing is used to introduce the exothermicreaction component and the acid precursor to the wellbore. In at leastone embodiment, the exothermic reaction component includes NH₄Cl andNaNO₂. The acid precursor is acetic acid. The acetic acid is mixed withNH₄Cl and injected in parallel with the NaNO₂, using different sides ofthe dual-string coiled tubing. The exothermic reaction component and theacid precursor mix within the fractures.

EXAMPLES Example 1

An exothermic reaction component of a cleanup fluid consisting of 3MNH₄Cl and 3M NaNO₂ was added to a solution of 1% by volume guar at roomtemperature, see FIG. 3. The exothermic reaction component was triggeredby heat. The viscosity of the solution was measured before, during, andafter the reaction using a Chandler viscometer. Prior to reaction of theexothermic reaction component, the viscosity of the residual viscousmaterial was 85 centipoise (cP). FIG. 1 is a graph of the viscosityfollowing the reaction of the exothermic reaction component. The graphshows that the viscosity of the residual viscous material was reduced toless than 8.5 cP. FIG. 3b shows the solution, including the residualviscous material after the reaction of the exothermic reactioncomponent.

Example 2

A solution of an exothermic reaction component was prepared from 3MNH₄Cl and 3M NaNO₂. The solution was placed in an autoclave reactor atroom temperature and an initial pressure of 1,000 psi. The reactortemperature was increased. The reaction was triggered at about 120° F.,see FIG. 2. Due to the reaction, the temperature in the reactor reacheda temperature of 545° F. and a pressure of 3,378 psi, see FIG. 2.

Example 3

The exothermic reaction component showed compatibility with the viscousfluid component (here a cross-linked gel). The fracturing fluid with theviscous fluid component, the exothermic reaction component, and theproppant component was also prepared and showed compatibility. Thefracturing fluid, without the proppant component, was activated in theautoclave reactor by heating to the wellbore temperature to trigger thereaction of the exothermic reaction component. The heat generated by thereaction reduced the viscosity of the viscous fluid component to producea reduced viscosity material, without injecting the viscosity breaker.Using a chandler viscometer, the viscosity of the fracturing fluid,containing the viscous fluid component and the exothermic reactioncomponent, was measured pre-reaction and post-reaction. The viscosity ofthe fracturing fluid was reduced from 1600 cP to 10 cP, as shown in FIG.4. The results show that the exothermic reaction component and this typeof treatment can clean-up the fractures post a fracturing job.

Example 4

An exothermic reaction component was applied to treat an injection well(also referred to as the wellbore) and the surrounding rock matrix inthe Safaniya Oil Field of Saudi Arabia. The injection well was damageddue to the deposition of asphaltenes and corrosion products in thewellbore and in the surrounding rock matrix. A laboratory analysis ofthe blockage materials showed the main components to be asphaltenes andcorrosion products, as shown in Table 1.

TABLE 1 Composition of blockage materials in injection well in SafaniyaOil Field. Component Weight Percent Asphaltene 11.3 Iron Oxides 32.4Iron Sulfides 21.6 Sodium Chloride 16.5 Calcium Carbonate 9.9 Silica 8.3

FIGS. 5a and 5b are pictorial representations of the blockage materialscollected from the injection well. In laboratory testing of the blockagematerials, the initial viscosity at 60° F. was about 5,800 cP. TheAmerican Petroleum Institute (API) number for the thick oil sludge was11. The blockage materials were a semi-solid material. The exothermicreaction component reduced the viscosity of the blockage material fromits initial viscosity and enabled easy cleanup.

FIG. 6 is a graph showing the effect of the exothermic reactioncomponent on the temperature of blockage materials shown in FIGS. 5a and5b . As can be seen, once an exothermic reaction component comprisingsodium nitrite and ammonium chloride was added to the blockage materialsat 127° F. at about the 50 second marker, the temperature increased toover 200° F.

FIG. 7 is a graph showing the effect of an exothermic reaction componenton the viscosity of blockage materials in Example 4, shown in FIGS. 5aand 5b . Surprisingly and unexpectedly, the viscosity of the blockagematerials (thick oil sludge) was quickly reduced from about 6,000 cP tounder 1,000 cP. In the lab-scale experiment, which used 10 grams of oilsludge material, 10 mL of 3M ammonium chloride and 10 ml of 3M sodiumnitrite were added to the oil sludge material along with 2 ml ofactivator (100% acetic acid).

FIG. 8 is a graph showing the effect of an exothermic reaction treatmentused to treat the blocked wellbore of Example 4. As shown by the graph,pressure and temperature within the wellbore increase with time. FIG. 9is a graph showing the effect of an exothermic reaction treatment usedto treat the rock matrix surrounding the wellbore of Example 4. As shownby the graph, pressure and temperature within the rock matrix increasewith time. Before the treatment of the wellbore and the surrounding rockmatrix with an exothermic reaction component, the injectivity of thewellbore was 1,000 barrels of water per day (1.0 MBWD). After theexothermic reaction treatment was used to treat the wellbore and thesurrounding rock matrix, the injectivity was improved by about 6 timesor about 600% to 5.8 MBPD.

FIG. 10 is a graph showing the pre-exothermic-reaction-componentinjectivity and post-exothermic-reaction-component injectivity of theinjection well of Example 4. As demonstrated in the graph, injectivitywas increased greatly over time by the treatment with the exothermicreaction component. WHP is wellhead pressure, and BPM is barrels perminute. Comparing the WHP when pumping 1 BPM before the exothermicreaction component to afterward, injectivity was improved by about 6times. WHP decreased from about 1,500 psi to about 253 psi. Theexothermic reaction component was able to remove asphaltenes, corrosioncomponents, scale, thick oil sludge, and other materials blocking theinjection wellbore and surrounding rock matrix.

Injectivity Test #'s 1 and 2 were conducted before the injection of theexothermic reaction component. Injectivity Test #3 was conducted afterthe addition of 10 bbls of reagent A (sodium nitrite) and 10 bbls ofreagent B (ammonium chloride and acetic acid), the reagents A and Bprepared as described as follows. Before Test #4, an additional 40 bblsof each reagent A and reagent B were injected into the wellbore,followed by soaking time. Then Test #4 was conducted. Test #'s 5 and 6were conducted after the addition of another 150 bbls of each reagent Aand 150 bbls of reagent B.

To prepare sodium nitrite (reagent A) for the application of theexothermic reaction component to the injection well, first 174 barrels(bbls) of fresh water were placed in a clean tank. Then, 13,165kilograms (kg) of sodium nitrite, from Bayouni Trading Co. of SaudiArabia, were added to the fresh water under agitation. Additional freshwater was mixed to balance to a total volume of solution of 200 bbls(however, this step was optional). To prepare ammonium chloride (reagentB) for the application of the exothermic reaction component to theinjection well, first 170 bbls of fresh water were placed in a cleantank. Then 10,313 kilograms (kg) of ammonium chloride, from BayouniTrading Co. of Saudi Arabia, were added to the tank under agitation.

Next, 20 bbls of acetic acid, from Schlumbergeer of Houston, Tex., wereadded to the tank with the ammonium chloride. In addition, 25 gallons ofcorrosion inhibitor were added, and fresh water was added to achieve atotal volume of solution of 200 bbls (however, this step was optional,and corrosion inhibitor is optional). The components were mixedthoroughly until all components were dissolved.

The concentration of ammonium chloride in solution and sodium nitrite insolution was about 3M. The concentration of acetic acid once mixed withthe ammonium chloride solution was about 5% by volume. Using the abovemethod to make sodium nitrite and ammonium chloride solutions, threeseparate runs were carried out on the well. In a first run, 10 barrelsof the sodium nitrite solution and 10 barrels of the ammonium chloridesolution were added (before Injectivity Test #3 in FIG. 10). In a secondrun, 40 barrels of the sodium nitrite solution and 40 barrels of theammonium chloride solution were added (before Injectivity Test #4 inFIG. 10). In a third run, 150 barrels of the sodium nitrite solution and150 barrels of the ammonium chloride solution were added (beforeInjectivity Test #'s 5 and 6 in FIG. 10).

Once the sodium nitrite and ammonium chloride were prepared, thesolutions were introduced to the injection well by coiled tubing toproduce an exothermic reaction in situ and reduce the viscosity of theblockage materials in the well and surrounding rock matrix. In thepresent example, HCl was not used as an activator. In some embodiments,HCl can be used as an activator in addition to or alternative to foracid stimulation, however, it was not used during the presentlydescribed well treatment.

The exothermic reaction can be activated by acid precursors such as, forexample, organic acids like acetic acid and inorganic acids such ashydrochloric acid. Such acid precursors can be encapsulated. Additionalprecursors can include organic esters.

Additional surprising and unexpected benefits using treatments of thepresent disclosure are observed. When fracturing low pressurereservoirs, usually fracturing takes a long time to flow back fracturinggel, which may take, for example, about 1-2 weeks. However, whencompositions of the present disclosure are used, well head pressure isincreased due to generated nitrogen gas in situ. Therefore, thisaccelerates the flow back time by just bleeding the pressure. As for theabove treatment, well head pressure increased, when the compositions ofthe present disclosure were added from about 2,600 to about 4,400 psi(third run from above, see also FIG. 9). Nitrogen gas generated by theexothermic reaction downhole provides lifting energy to accelerate flowback from the well. Flow back can be reduced from about 1-2 weeks toabout 1-2 hours.

Additional and different reactive chemicals that generate additionalheat or pressure downhole can be used in addition to or alternative tothe exothermic reaction component to enhance the efficiency. Certainexample compounds include sodium azide and ammonium nitrite. Suchchemicals could be used in addition to or alternative to sodium nitrite,ammonium chloride, and an activator.

Additional reactive chemicals with triggering temperatures at about 300°C. to about 400° C. could be used, where the heat generated by thereaction between ammonium chloride and sodium nitrite would provideactivation energy for other reactive chemicals downhole.

Although the present disclosure has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereupon without departing from the principle and scope of thedisclosure. Accordingly, the scope of the present disclosure should bedetermined by the following claims and their appropriate legalequivalents.

The singular forms “a,” “an,” and “the” include plural referents, unlessthe context clearly dictates otherwise.

Optional or optionally means that the subsequently described event orcircumstances can or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed here as from about one particular value, or toabout another particular value. When such a range is expressed, it is tobe understood that another embodiment is from the one particular valueto the other particular value, along with all combinations within saidrange.

Throughout this application, where patents or publications arereferenced, the disclosures of these references in their entireties areintended to be incorporated by reference into this application, in orderto more fully describe the state of the art to which the disclosurepertains, except when these references contradict the statements madehere.

As used throughout the disclosure and in the appended claims, the words“comprise,” “has,” and “include” and all grammatical variations thereofare each intended to have an open, non-limiting meaning that does notexclude additional elements or steps.

As used throughout the disclosure, terms such as “first” and “second”are arbitrarily assigned and are merely intended to differentiatebetween two or more components of an apparatus. It is to be understoodthat the words “first” and “second” serve no other purpose and are notpart of the name or description of the component, nor do theynecessarily define a relative location or position of the component.Furthermore, it is to be understood that that the mere use of the term“first” and “second” does not require that there be any “third”component, although that possibility is contemplated under the scope ofthe present disclosure.

What is claimed is:
 1. A cleanup fluid for reducing a viscosity of a viscous material in and around a wellbore of a hydrocarbon-bearing formation, the cleanup fluid comprising: an acid precursor, the acid precursor operable to trigger an exothermic reaction component; and the exothermic reaction component operable to generate heat, where the heat is operable to reduce a viscosity of the viscous material to create a reduced viscosity material, the reduced viscosity material operable to flow to allow increased fluid flow in the wellbore.
 2. The cleanup fluid of claim 1, where the exothermic reaction component comprises an ammonium containing compound and a nitrite containing compound.
 3. The cleanup fluid of claim 2, where the ammonium containing compound comprises NH₄Cl and the nitrite containing compound comprises NaNO₂.
 4. The cleanup fluid of claim 3, where the concentration of the NH₄Cl and NaNO₂ is about 3 M.
 5. The cleanup fluid of claim 1, where the acid precursor is selected from the group consisting of triacetin, methyl acetate, hydrochloric acid, acetic acid, and combinations thereof.
 6. The cleanup fluid of claim 1, where the acid precursor comprises triacetin.
 7. The cleanup fluid of claim 1, where the exothermic reaction component reacts when a temperature within the hydrocarbon-bearing formation reaches about 120° F.
 8. The cleanup fluid of claim 1, further comprising a cross-linked gel.
 9. The cleanup fluid of claim 1, further comprising a viscous fluid component and a proppant component.
 10. The cleanup fluid of claim 9, where the viscous fluid component comprises a compound selected from the group consisting of: carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methyl hydroxyl ethyl cellulose, hydroxypropyl guar, carboxymethyl guar, guar cross-linked boron ions from an aqueous borax/boric acid solution, guar cross-linked with organometallic compounds, aluminum phosphate-ester oil gels, and mixtures thereof.
 11. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 0.1% and about 15% by volume.
 12. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 0.1% and about 10% by volume.
 13. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 1% and about 10% by volume.
 14. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 2% and about 8% by volume.
 15. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 4% and about 6% by volume.
 16. The cleanup fluid of claim 1, further comprising an ammonium containing compound selected from the group consisting of: ammonium chloride, ammonium bromide, ammonium nitrate, ammonium sulfate, ammonium carbonate, ammonium hydroxide, and mixtures thereof.
 17. The cleanup fluid of claim 1, further comprising a nitrite containing compound selected from the group consisting of: sodium nitrite, potassium nitrite, and mixtures thereof.
 18. A method for use of the cleanup fluid of claim 1, the method comprising the steps of: injecting the acid precursor and the exothermic reaction component into the wellbore; allowing the exothermic reaction component to react in situ to produce heat and nitrogen gas, the heat and nitrogen gas operable to increase the temperature and pressure in situ; and reducing the viscosity of the viscous material to create the reduced viscosity material.
 19. The method of claim 18, where the exothermic reaction component comprises an ammonium containing compound and a nitrite containing compound.
 20. The method of claim 19, where the ammonium containing compound comprises NH₄Cl and the nitrite containing compound comprises NaNO₂.
 21. The method of claim 18, where the acid precursor is selected from the group consisting of triacetin, methyl acetate, hydrochloric acid, acetic acid, and combinations thereof.
 22. The method of claim 18, where the exothermic reaction component comprises an ammonium containing compound selected from the group consisting of: ammonium chloride, ammonium bromide, ammonium nitrate, ammonium sulfate, ammonium carbonate, ammonium hydroxide, and mixtures thereof.
 23. The method of claim 18, where the exothermic reaction component comprises a nitrite containing compound selected from the group consisting of: sodium nitrite, potassium nitrite, and mixtures thereof.
 24. The method of claim 18, where the viscous material comprises asphaltenes and corrosion products in the wellbore.
 25. The method of claim 24, where the corrosion products are selected from the group consisting of: iron oxides, iron sulfides, sodium chloride, calcium carbonate, silica, and mixtures thereof.
 26. The method of claim 18, wherein the step of reducing the viscosity of the viscous material to create the reduced viscosity material increases the injectivity of the wellbore.
 27. The method of claim 18, wherein the step of reducing the viscosity of the viscous material to create the reduced viscosity material increases the productivity of the hydrocarbon-bearing reservoir.
 28. The method of claim 18, further comprising the step of increasing nitrogen lift in the wellbore. 